The invention relates to a method for energizing a superconducting magnet arrangement which is disposed in a cryostat device for cooling to a cryogenic operating temperature and which has a first magnet winding comprising a tape conductor with a first superconductor material and a first transition temperature above the operating temperature, the method comprising the following steps:    (a) temperature-controlling of the magnet arrangement to a first pre-operating temperature between the first transition temperature and the operating temperature,    (b) excitation of a first pre-operating current after the first pre-operating temperature is reached at least in a part of the first magnet winding,    (c) cooling the magnet arrangement to the operating temperature,    (b) excitation of a first operating current at least in a part of the first magnet winding at the latest after the operating temperature is reached, wherein the first magnet winding with the first operating current generates a first operating magnetic field in the volume of the first magnet winding.
Such a method is known from the publication by Y. Yanagisawa et al., “Effect of coil current sweep cycle and temperature change cycle on the screening current-induced magnetic field for YBCO-coated conductor coils”, AIP Conference Proceedings 1434, 1373 (2012) (=Reference [9]).
Many magnetic resonance methods require static magnetic fields which are as homogeneous as possible, for example in order to achieve a high spectral resolution in the case of a spectroscopic method or in order to achieve sharp imaging which is as free of distortion as possible in the case of an imaging method. Superconducting magnet arrangements are usually employed for field generation.
After charging of such a superconducting magnet arrangement with tape conductors, in particular high-temperature superconductors (=HTS), partially persistent screening currents can flow in the tape which reduce the field strength of the magnet arrangement, or lead to field inhomogeneities and field instability. This is described in detail for example in reference [1].
Different methods and arrangements for cancellation of screening currents on the charged magnet are known. These are based on the following ideas: Running through field cycles, high-frequency field excitations, optimization of the coil design (design relating to coil shape, conductor geometry, current-carrying capacity of the superconductor).
There is also the idea of not eliminating the screening currents, but taking the effects thereof into consideration in the design of the magnet arrangement and providing corresponding possibilities for field correction (for example shim coils).
The prior art documents cited in the list of references given at the end of the description disclose the following aspects which are fundamental as the starting point for the present invention:    [1] magnet arrangement with tape-like HTS windings, e.g. for high-field NMR (nuclear resonance) applications. After charging of HTS tape conductors partially persistent screening currents flow which reduce the field strength of the magnet arrangement or lead to field inhomogeneity and field instability.
[2] magnet arrangement with HTS and LTS (=low-temperature superconductor) windings, for example for high-field NMR applications. Due to overcurrent cycles in the HTS part, the field drift is minimized after the final current is reached.
[3] “longitudinal vortex shaking” by a high-frequency magnetic field in an HTS magnet winding. The high-frequency field is perpendicular to the static field of the HTS magnet winding and parallel to the tape direction.
[4] “transverse vortex shaking” by a high-frequency magnetic field in an HTS magnet winding. The high-frequency field is perpendicular to the static field of the HTS magnet winding and transverse with respect to the tape direction.
[5] “vortex shaking” by a high-frequency magnetic field in an HTS magnet winding. Arrangements of copper coils for generation of the high-frequency field are shown.
[6] Multifilament HTS tape conductors reduce the undesirable field contributions of the screening currents. Overcurrent cycles for stabilization of the field drift and reduction of the field inhomogeneity due to screening currents. Calculation of the screening current intensities as a function of the form factor of the HTS winding.
[7] Calculation of the screening current intensities in an HTS winding as a function of the coil shape, conductor width and the critical current of the superconductor. The optimum is a narrow conductor, small coil diameter, and substantial operating current in relation to the critical current.
[8] Measurement of the effect of screening currents in an arrangement of a plurality of coaxial HTS pancake coils at a temperature of 77 K in the self-field and/or in a background field. Verification of a simulation method for the design of field homogenization coils (shims) for compensation for the field inhomogeneity due to the screening currents in the HTS winding in particular in a magnet arrangement with HTS and LTS windings for high-field NMR applications.
[9] Reduction of the screening currents by field and temperature cycles. The effect of the temperature cycles on the screening currents in an HTS winding (YBCO) is studied experimentally in the range from 77 to 82 K and proposed for cryocooler-cooled magnet arrangements (see for instance page 1379: “This kind of coil temperature cycle is available for cryocooler cooled YBCO magnets.”).                This prior art according to reference [9]—already cited above—is the closest to the intellectual starting point of the present invention:        After charging of a magnet arrangement, the temperature is temporarily increased, so that the screening currents are eliminated to some extent. This condition is then “frozen” at the operating temperature with further cooling.        
However, until now this method has only been known for pure HTS magnet arrangements. If a part of the magnet arrangement comprises a low-temperature superconductor, this method could not be used hitherto because with it the charged magnet would be heated above the transition temperature of the low-temperature superconductor, which would inevitably result in “quenching” of the system, that is to say a breakdown of the superconducting currents in the magnet arrangement.
The object of the present invention is to provide a method of the type defined in the introduction, in which upon charging of superconducting magnet arrangements with combined magnet windings made of superconducting materials having significantly different transition temperatures, in particular in the case of systems with low-temperature superconductors (LTS) and high-temperature superconductors (HTS), elimination or prevention of screening currents by temperature-controlling of the magnet arrangement as well as subsequent freezing of the condition with reduced screening currents is achieved upon cooling to the operating temperature, without eliminating currents in the magnet windings having the material with a lower transition temperature (in particular LTS material).